Method and apparatus for radiation assisted electrochemical etching and etched product

ABSTRACT

An electrochemical etching system has an etching bath for holding an n-type silicon substrate with a first surface of the substrate in contact with hydrofluoric acid, an electrode positioned in the hydrofluoric acid, a power source having a positive pole connected to the silicon substrate and a negative pole connected to the electrode, and an illumination unit having a light source for illumination of a second surface of the silicon substrate. The illumination unit illuminates the second surface of the silicon substrate with an illumination intensity of 10 m W/cm 2  or more. A ratio of a maximum illumination to a minimum illumination of the second surface of the silicon substrate is 1.69:1 or less. With the etching system, pores and/or trenches of a certain size and shape can be formed in an entire area of the silicon substrate having a diameter of more than three inches.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an electrochemical etching method andapparatus. In particular, the present invention relates to a method andapparatus in which an n-type silicon substrate is exposed at one surfaceto an electrolyte and at an opposite surface to light, so that a pore(hole) or a trench (groove) of a certain size and shape is formed in thesubstrate as an etching current flowing in the substrate is controlledby the light. Also, the present invention relates to a product, e.g., asemiconductor device, made by the use of the electrochemical etchingmethod. It is to be understood that the present invention is preferablyapplicable to a method and apparatus for an electrochemical etching forthe formation of pores or trenches having a diameter or width of 50 nmor more in the n-type silicon substrate. However, the present inventionis not limited by the size of the pore or trench.

BACKGROUND OF THE INVENTION

Japanese Patent Publication No. 2,694,731 discloses an electrochemicaletching system which uses light to form small pores or trenches in ann-type doped silicon substrate. The system has a holder for holding then-type doped silicon substrate (silicon wafer) with one surface of thesubstrate contacted with an electrolyte (hydrofluoricacid). Also, theholder retains an electrode in the electrolyte so that the electrodeopposes the silicon substrate. With this etching system, the siliconsubstrate is positively biased and the electrode in the electrolyte isnegatively biased. The opposite side of the silicon substrate away fromthe electrolyte is exposed to light, causing holes in he siliconsubstrate. The holes travel a boundary region between the siliconsubstrate and the electrolyte to resolve the boundary portion of thesilicon substrate. This means that an arrangement of a masking barrier(coating) with one or more apertures (pits) on the surface of thesilicon substrate adjacent to the electrolyte, results in the formationof the pores or trenches in the substrate portions, corresponding to theapertures.

The Journal of Electrochemical Society, No. 140, October 1993, pp.2836-2843 discloses a back light device for the illumination of siliconsubstrate. The light device has a lamp for emitting light, an infraredfilter for removing infrared light from the emitted light, and a convexlens for collimating the light emitted from the lamp.

Also, the Journal of Electrochemical Society, No. 137, February 1990,pp. 653-659 discloses an electrochemical etching device which uses a 100W tungsten lamp for the back light device.

Further, Japanese Patent Publication No. 11-509644 discloses a systemfor manufacturing devices with electrochemical etching. Japanese PatentPublication No. 11-154737 discloses a manufacturing system forincorporating a capacitance in the trench formed by the electrochemicaletching technique. The Journal of Electrochemical Society, No. 137,February 1990, pp. 653-659 discloses an embodiment in which an apertureor trench of 20 ×20 mm is formed in the silicon substrate by the etchingtechnique.

In order to mass-produce various devices using the electrochemicaletching system, the system is required to make an even etching for theentire surface of a relatively large silicon substrate with a diameterof three inches or more, for example, and thereby form pores or trenchesof a certain size and shape at every portion of the substrate.

Using the system disclosed in the Japanese Patent Publication No.2,694,731 and the 100 W tungsten lamp in the Journal of ElectrochemicalSociety, No. 140, trials were made to form pores of a certain diameterat every portion in the three-inch silicon substrate. The etched siliconsubstrate was viewed by the microscope, which showed that pores wereformed only in a limited part of the silicon substrate. Also, theresultant pores have different sizes and shapes. Although further trialswere made under different conditions in voltage, current andillumination, uniform pores failed to be formed over the entire portionof the silicon substrate.

Another object of the present invention is to provide devices, e.g.,semiconductor device and sensors, such as acceleration sensor,manufactured through such electrochemical etching method.

SUMMARY OF THE INVENTION

In order to attain the objects, an electrochemical etching systemaccording to one aspect of the present invention has an illuminationunit including a light source for illuminating an illumination surfaceof an n-type silicon substrate with an illumination of 10 mW/cm² ormore. According to the embodiment, even for the silicon substrate havinga diameter of three inches or more, pores and/or trenches to be formedin the silicon substrate develops toward the illumination surface with auniform cross section. Also, the formed pore and/or trench has a smoothsurface.

In another aspect of the electrochemical etching system of the presentinvention, on the illumination surface of the silicon substrate, a ratioof the maximum illumination to the minimum illumination is 1.69:1 orless. With the arrangement, even for the silicon substrate having adiameter of three inches or more, a constant etching current flows inthe silicon substrate, which ensures that the formed pore and/or trenchhas a substantially constant size (cross section and depth).

In another aspect of the electrochemical etching system of the presentinvention, a reference electrode is positioned in the electrolyte. Avoltage detector with an elevated impedance is electrically connectedbetween the reference electrode and the n-type silicon substrate. Withthe arrangement, by controlling the voltage between the referenceelectrode and the silicon substrate, the voltage to be applied to thesilicon substrate can be controlled.

In another aspect of the electrochemical etching system of the presentinvention, an illumination unit has an illumination controller forcontrolling an illumination to the other surface of the siliconsubstrate. With the arrangement, the size of the pore and/or trench tobe formed in the silicon substrate can be controlled.

In another aspect of the electrochemical etching system of the presentinvention, the illumination controller controls an amount of lightemitted from the light source. With the arrangement, the illumination tothe silicon substrate can be adjusted precisely.

In another aspect of the electrochemical etching system of the presentinvention, the illumination controller, which is positioned between thelight source and the silicon substrate, has a modulator for modulatinglight emitted from the light source. With the arrangement, where thelight source is unable to control an amount of light to be emittedtherefrom, the illumination of the silicon substrate can be controlled.

In another aspect of the electrochemical etching system of the presentinvention, the system includes a current detector for detecting acurrent applied from the power source to the silicon substrate, and acircuit for controlling the emitting light according to the currentdetected by the current detector. With the arrangement, the siliconsubstrate can be etched precisely.

In another aspect of the electrochemical etching system of the presentinvention, the system includes a unit for maintaining a stable conditionof the hydrofluoricacid (e.g., concentration and temperature). With thearrangement, the hydrofluoricacid has a stable condition, which in turnensures the constant size of the pore and/or trench formed in thesilicon substrate.

In another aspect of the electrochemical etching system of the presentinvention, the system has a metal plate positioned on the other surfaceof the silicon substrate. The metal plate is formed with a number ofregularly arranged openings so that light from the illumination unittoward the silicon substrate is transmitted therethrough. With thearrangement, the other surface of the silicon substrate is illuminateduniformly, which ensures the silicon substrate to be applied with aconstant voltage.

In another aspect of the electrochemical etching system of the presentinvention, the metal plate is made of electrically conductive materialand is positioned adjacent to the other surface of the siliconsubstrate. With the arrangement, the power source and the siliconsubstrate are electrically connected through the metal plate.

In another aspect of the electrochemical etching system of the presentinvention, the metal plate is formed integrally on the other surface ofthe silicon substrate. With the electrochemical etching system, themetal plate is formed precisely by the physical or chemical vapordeposition and also the lithography used in the manufacturing processfor semiconductor. Also, the openings can be formed with a greatprecision.

In another aspect of the electrochemical etching system of the presentinvention, the metal plate is formed independent of the siliconsubstrate. With the arrangement, the manufacturing process of thesilicon substrate can be simplified.

An electrochemical etching method of the present Invention whichincludes the steps of making one surface of an n-type silicon substrateinto contact with an electrolyte, illuminating the other surface of thesilicon substrate, and controlling an etching current by theillumination to form a pore or trench in the one surface of the siliconsubstrate is characterized in that the method further comprisesilluminating the the other surface of the silicon substrate with anillumination of 10 mW/cm² or more.

In another aspect of the electrochemical etching method, the methodincludes arranging a metal plate with a number of regularly arrangedopenings on the other surface of the n-type silicon substrate andilluminating the other surface of the n-type silicon substrate throughthe openings.

In another aspect of the electrochemical etching method, a ratio of amaximum illumination to a minimum illumination to the other surface ofthe silicon substrate is 1.69:1 or less.

With the methods, even the silicon substrate having a diameter of threeinches or more is formed with a substantially the same size pores and/ortrenches in an entire area of the substrate.

Another electrochemical etching method of the present invention whichhas the steps of making one surface of an n-type silicon substrate intocontact with an electrolyte, illuminating the other surface of thesilicon substrate, and controlling an etching current by theillumination to form pores or trenches in the one surface of the siliconsubstrate is characterized in that the method further comprises a firststep in which the other surface of the silicon substrate is illuminatedwith a first illumination of 10 mW/cm² or more to form the pores ortrenches extending toward the other surface of the silicon substrate,and a second step in which, after the first step, the other surface ofthe silicon substrate is illuminated with another illumination higherthan the first illumination to extend the pores or trenches laterally toconnect the pores or trenches to each other. According the method, thevertical pores can be connected at bottom portion thereof to each other.

In view of above, according to the electrochemical etching method of thepresent invention, the shapes of the pores and/or trenches can becontrolled so precisely. Also, an enlarged substrate can be etched.Then, the devices manufactured by the electrochemical etching system haspores and/or trenches of which shape is well controlled, ensuring a highperformance and its inexpensiveness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of an electrochemical etchingsystem according to the first embodiment;

FIGS. 2A to 2E are drawings, each of which showing a cross sectionalshape of a pore formed in the silicon substrate, a diameter of the pore,and an illumination in Experiment 2;

FIG. 3 is an exaggerated plan view of a grid electrode layer positionedon a back surface of the silicon substrate;

FIG. 4 is an enlarge cross sectional view taken along lines IV—IV inFIG. 3, showing the grid electrode layer in an exaggerated fashion;

FIG. 5 is a schematic cross sectional view of the electrochemicaletching system according to the second embodiment;

FIG. 6 is a schematic cross sectional view of the electrochemicaletching system according to the third embodiment;

FIG. 7 is a schematic cross sectional view of the electrochemicaletching system according to the fourth embodiment;

FIG. 8 is a schematic cross sectional view of the electrochemicaletching system according to the fifth embodiment;

FIG. 9 is a schematic cross sectional view of the electrochemicaletching system according to the sixth embodiment;

FIGS. 10A and 10B are a schematic plan view and a detail view of thegrid electrode plate for use in the electrochemical etching systemaccording to the seventh embodiment;

FIG. 11 is a schematic plan view of an acceleration sensor manufacturedby the electrochemical etching method according to the eighthembodiment;

FIGS. 12A to 12E are drawings for describing processes of theelectrochemical etching method according to the eighth embodiment;

FIG. 13 is a perspective view of an optical guide member manufactured bythe electrochemical etching method according to the ninth embodiment;and

FIGS. 14A to 14E are drawings for describing process of theelectrochemical etching method according to the ninth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, descriptions will be made to thepreferred embodiments of the present invention. Like reference numeralsindicate like parts throughout the drawings.

First Embodiment

Referring to FIG. 1, there is shown an electrochemical etching systemfor an n-type silicon substrate (silicon wafer) 10 according to thefirst embodiment of the present invention. The etching system 10includes an etching bath 12 for receiving an etching electrolyte 14 of5wt % hydrofluoricacid. A surface portion of the etching bath 12, makinga contact with the etching electrolyte 14, is coated with an appropriatematerial, e.g., polytetrafluoroethylene, that resists againsthydroflluoricacid. Alternatively, the etching bath 12 may fully be madeof material that resists against hydroflluoricacid.

The etching bath 12 is formed at its wall 16 with a round opening 18 forreceiving a disc-like n-type silicon wafer 20 therein. The wall 16includes an annular flange 22 running along an inner periphery of theopening 18 and projecting toward a center of the opening 18. In order tosecure the silicon substrate 20, a fixing ring 24 is provided behind thesubstrate 20 to force the silicon substrate 20 against the annularflange 22. Preferably, a suitable sealing member, e.g., O-ring 26, ispositioned between the annular flange 18 and the silicon substrate 20 toprevent the electrolyte 14 from leaking therebetween.

One electrode 28 (cathode) is provided in the electrolyte 14 so that itopposes to the silicon substrate 20 received in the opening 18. Thesilicon substrate 20 is used for the other electrode or anode. Theelectrode (cathode) 28 and the silicon substrate (anode) 20 areelectrically connected to a DC power supply 30, so that a suitablevoltage can be applied between the electrode (cathode) 28 and thesilicon wafer (anode) 20. Preferably, the fixing ring 24 for fixing thesilicon substrate 20 in position is made of an electrically conductivematerial for the electrical connection between the silicon substrate 20and the power source 30.

A surface 32 of the silicon wafer 20 adjacent to the electrolyte 14 iscoated with a masking barrier or resist mask 34. The masking barrier 34is made of a suitable masking material, e.g., silicon nitride, platinum,or gold, which is deposited by a suitable deposition technique, e.g.,chemical or physical vapor deposition. The surface 32 of the siliconsubstrate is formed with one or more pits or apertures 36 correspondingto an etching pattern of the silicon substrate 20. This causes thatetching or resolving proceeds at the associated portions of the siliconsubstrate exposed in the pits 36 toward the opposite surface 38 of thesilicon substrate, forming pores or trenches in silicon substrate.Preferably, the pits 36 are formed by a suitable technique such as wetetching, dry etching, and laser machining.

The etching system 10 includes an electrolyte unit 40. The electrolyteunit 40 has a circulation passage or tube 42 which is fluidly connectedat its opposite ends to the etching bath 12. The circulation passage 42has a pump 44 for pumping the electrolyte 14 through the circulationpassage 42, a filter 46 for removing foreign substances from theelectrolyte 14 running in the circulation passage 42, a buffer 48 formaintaining a constant amount of electrolyte 14 in the etching bath 12,and a thermostat 50 for maintaining a temperature of electrolyte 14 inthe etching bath 12 constant. The electrolyte unit 40 maintains anecessary quality of the electrolyte 14 in the etching bath 14, ensuringa stable etching of the silicon substrate 20 described below. Theetching system 10 further includes a back light unit 52. The back lightunit 52 is used to concentrate holes generated in the silicon substrate20 at a leading edge of each pore where the further etching or resolvingof the substrate proceeds locally. For this purpose, the back light unit52 has an illumination source or lamp 56, e.g., tungsten lamp, locatedon an central axis 54 of the circular opening 18 of the etching bath 12and a semi-oval reflector or mirror 58 of which opening 60 is directedtoward the circular opening 18 of the etching bath 12. A curvature ofthe mirror 58 is determined so that light from the lamp 54 is reflectedby the mirror 58 to focus on a certain point (focal point) 62 on theaxis 54. A collimator lens 64 is opposed to the opening 60 of the mirror58 so that light from the mirror 58 is collimated by the collimator lens64. Another convex lens 66 is positioned between the collimator lens 64and the silicon substrate 20 so that the collimated light is thenextended toward the silicon substrate 20, causing the back surface ofthe silicon substrate, away from the electrolyte, to be illuminated.Preferably, as shown in FIG. 1, a filter 68 is provided to remove a partof light, e.g., light having a wavelength of 1.1 μm or more, in order toprevent the silicon substrate 20 from being overheated. Also, a suitablefan or fans 70 may be provided adjacent to the silicon substrate 20 forthe cooling of the substrate.

Hereinafter, a brief description will be made to the operation of theetching system 10 so constructed. In operation, the silicon substrate 20is secured in the opening 18 of the etching bath 12. Then, theelectrolyte 14 is filled in the etching bath 12. Then, the power source30 applies a certain voltage between the electrode 28 and the siliconsubstrate 20, and the illumination lamp 52 illuminates the back surface38 of the silicon substrate 20. This causes holes in the siliconsubstrate to concentrate at the portions of the silicon substrate,exposed in the pits 36. As a result, a local etching or resolving isinitiated at each of the exposed portions and then advanced straightlytoward the opposite back surface 38 of the silicon substrate 20.

Experiment 1

Tests were made to evaluate a relation between an illumination andconfigurations of the resultant pores.

1. Conditions

Test conditions were as follows:

i. Silicon substrate

Diameter of silicon substrate: 76 mm

Thickness of silicon substrate: 625 μm

Thickness of mask barrier: 5,000 Å

Diameter of pit: 2 μm

ii. Electrolyte

5wt % hydrofluoricacid

iii. Illumination

0, 5, 10, 20, 50, 100, and 200 mW/cm²

The illumination was measured on the back surface of the siliconsubstrate, using a Power Meter commercially available from AdvantestCorporation under the name of Air Light Multi-Power Q8221, with acalibrated wavelength of 760 nm.

iv. Voltage (Voltage applied between the electrode and the siliconsubstrate)

2.0 volts and 4.0 volts

v. Etching period

20 minutes

2. Evaluation

The etched silicon was cut and the its cross section, in particular, thelongitudinal cross section of pore, was observed using a microscope.Using the microscopic photographs, diameters of several pores weremeasured at a certain distance of 5 μm away from the front surface ofthe silicon substrate.

3. Results

The test results are shown in the following Table 1:

TABLE 1 Illumination Voltage Applied (mW/cm²) 2.0 volts 4.0 volts 0 C C5 B B 10 A A 20 A A 30 A A 50 A A 100 A A 200 A A

Table 1, evaluation A, B and C means as follows:

A: Each pore had a diameter of more than 50 nm. Also, each pore had asmooth surface and extended substantially straightly toward the backsurface.

B: Each pore had a diameter of less than 50 nm. Also, each pore extendedobliquely toward the back surface.

C: No pore was formed.

4. Conclusion

The table 1 shows that the illumination affects on the resultant poreconfiguration significantly. Also revealed is that, in order to form apore running straightly toward the back surface of the substrate andhaving an even cross section, the illumination should be set 10 mW/cm²while keeping a suitable voltage between the electrode and the siliconsubstrate.

In order to provide the illumination of 10 mW/cm² for the entire backsurface of the silicon substrate, a source. Anther lamp such as tungstenlamp or mercury lamp can be used instead provided that it ensures theillumination of 10 mW/cm².

An application of an increased voltage between the silicon substrate andthe electrode may result in pores having a diameter of 50 nm or more inthe silicon substrate. However, this etching is different from theelectrochemical etching technique according to the present invention.

Experiment 2

Tests were made to evaluate an affect of illumination on theconfiguration of pores in the silicon substrate.

1. Conditions

Test conditions were as follows:

i. Silicon substrates

Number of substrates used: 4

Diameter of silicon substrate: 76 mm

Thickness of silicon substrate: 625 μm

Thickness of mask barrier: 5,000 Å

Diameter of pit: 2 μm

ii. Electrolyte

5 wt % hydrofluoricacid

iii. Illumination

The illumination was measured on the back surface of the siliconsubstrate. Then, identified were the maximum and minimum illuminationpoints of the substrate. The illumination measuring device was identicalto that used in Experiment 1.

iv. Voltage (Voltage applied between the electrode and the siliconsubstrate)

2.0 volts

V. Etching period

20 minutes

2. Evaluation

The etched silicon was cut and the its cross section, in particular, thelongitudinal cross section of pore, was observed using a microscope.Using the microscopic photographs, diameters of several pores weremeasured at a certain distance of 5 μm away from the front surface ofthe silicon substrate.

3. Results

The test results are shown in FIGS. 2A-2D. As shown in the tables in thedrawings, where a ratio (I_(MAX)/I_(MIN)) of the maximum illuminationI_(MAX) and the minimum illumination I_(MIN) being 1.96 and 2.25, theconfigurations of pores formed at the maximum illumination point and theminimum illumination point were quite different from each other. Also,the diameter of the pore at the maximum illumination was extended towardthe leading end of the pore.

Where the ratio of the maximum illumination and the minimum illuminationbeing 1.69, although the pores had separate diameters slightly differentfrom the other, they have substantially the same depth. Also, the poreshad substantially smooth surfaces. Likewise, where the ratio of themaximum illumination and the minimum illumination being 1.21, the poreshad substantially the same size and they had smooth surfaces.

4. Conclusion

The result of experiment 2 shows that, even for the silicon substratehaving a diameter of 3 inches or more, pore or trenches havingsubstantially the same diameter are formed at every place in the siliconsubstrate provided that the ratio of the maximum illumination and theminimum illumination is 1.69 or less.

Improvements/Modification

FIGS. 3 and 4 show the silicon substrate 20. The silicon substrate 20has an n+ layer 80 in the back surface 38 opposing to the back lightunit 52, in which ion is injected using a known donor ion injectiontechnique. A metal layer (metal plate or grid metal layer) 82, made ofconductive material, is provided on the n+ layer 80. Preferably, as bestshown in FIG. 3, the metal layer 82 is in the form of grid. Preferably,the grid metal layer 82 is made by depositing the conductive materialthrough a film formation technique such as chemical or physical vapordeposition and then forming openings 86 so that a grid 84 remainsbetween openings through a technique such as photolithography.

The grid layer 82 so defined has a smaller contact resistance againstthe n-type silicon substrate 20, which allows a constant voltage to beapplied through the grid metal layer 20 to the entire portion of thesilicon substrate 20 which is in contact with the grid metal layer 82.

Although the opening 86 in the grid 84 is not limited to a specificsize, it should be smaller than the thickness of the silicon substrate20. This is because that the illumination to the silicon substrate 20decreases with the increase of the width of the grid 84 between theopenings 86, which reduces the concentration of the hole in the siliconsubstrate 20.

Studies conducted by the inventors revealed that, where each line of thegrid 84 has a width of 10 μm and its interval is 90 μm, a constantvoltage can be applied to every portion of the silicon substrate 20,overcoming the problems caused by the reduction of illumination.

Preferably, when using electrode 28 made of platinum, the voltagebetween the electrode 28 and the silicon substrate 20 is determined sothat the n-type silicon substrate 20 is biased higher than the electrode28 by +1 to +4 volts. This causes pores and trenches to be generated inthe substrate more efficiently.

Second Embodiment

Referring to FIG. 5, there is shown another electrochemical etchingsystem 90 according to the second embodiment of the present invention.In this etching system 90, the back light unit 92 includes a pluralityof lamps 94 arranged in a lattice on a plane perpendicular to thecentral axis of the silicon substrate 20 (or on a plane parallel to thesilicon substrate 20). With this etching system 90, the back surface ofthe silicon substrate 20 is illuminated uniformly. Also, the pluralityof lamps 94 can be positioned easily so that the ratio ofmaximum/minimum illumination ratio takes 1.69 or less.

Third Embodiment

Referring to FIG. 6, there is shown another electrochemical etchingsystem 100 according to the third embodiment of the present invention.In this etching system 100, a reference electrode 102, which ispositioned in the electrolyte 14 between the silicon substrate 20 andthe electrode 28, is electrically connected through a voltage meter 104to the power source 30 for measuring the voltage to be applied to thesilicon substrate 20. Preferably, the voltage meter 104 is designed toprovide an elevated impedance between the reference electrode 20 and thesilicon substrate 20. In operation of the etching system 100, the powersource 30 is controlled so that a constant voltage is applied to thevoltage meter 104, ensuring a constant current to flow between thesilicon substrate 20 and the electrode 28.

Preferably, the reference electrode 102 is positioned as close aspossible to the silicon substrate 20 while leaving a small gaptherebetween. This minimizes the electric resistance of the electrolyte14 between the reference electrode 102 and the silicon substrate 20.Also, this ensures that the voltage applied to the silicon substrate 20is detected precisely and, thereby, that the configuration of thepore/trench is well controlled.

Fourth Embodiment

Referring to FIG. 7, there is shown another electrochemical etchingsystem 110 according to the fourth embodiment of the present invention.In this etching system 110, the illumination lamp 56 is electricallyconnected to a voltage controller 112 for changing the illumination tothe silicon substrate 20. With this etching system 110, an etchingcurrent supplied from the power source 30 to the silicon substrate 20changes in proportion to the illumination. Therefore, by changing thevoltage applied to the lamp 56 by the voltage controller 112 and,thereby, the illumination to the silicon substrate 20, the size of theresultant pore and/trench can be varied. Also, a pore and/or trench withan enlarged cavity or cross section at its leading portioin can easilybe formed by forming a pore or trench having a uniform cross section andthen increasing the illumination and the etching current.

Fifth Embodiment

Referring to FIG. 8, there is shown another electrochemical etchingsystem 120 according to the fifth embodiment of the present invention.This etching system 120 has two polarizing devices or filters 122 and124 between the convex lens 66 and the filter 68. Also, one of the twopolarizing filters 122 and 124 is supported for rotation about a centralaxis 54 of the silicon substrate 20 relative to the other in order tocontrol a quantity of light passing through the two polarizing filters122 and 124 and thereby the illumination to the silicon substrate 20.This means that simply by rotating the rotatable filter, rather thancontrolling the illumination of the lamp 56, the size of the pore andtrench formed in the silicon substrate can be controlled.

In this embodiment, the rotatable polarizing filter may be mechanicallyconnected to a drive unit (e.g., motor) 126 for rotating the polarizingfilter about the central axis 54, which in turn connected to thecontroller 128. This allows the controller 128 to control the drive unit126, positioning the polarizing filter in a desired position.

Sixth Embodiment

Referring to FIG. 9, there is shown another electrochemical etchingsystem 130 according to the fifth embodiment of the present invention.In this etching system 130, the illumination lamp 56 is electricallyconnected to a voltage controller 132 for controlling the voltageapplied to the lamp 56. A current detector or amperemeter 134 iselectrically connected between the power source 30 and the siliconsubstrate 20 for detecting the etching current supplied from the powersource 30 to the silicon wafer 20. The voltage controller 132 and theamperemeter 134 are electrically connected to each other through afeedback circuit 136.

With this etching system 130, the feedback circuit 136 reads the etchingcurrent detected by the amperemeter 134, and then transmits acorresponding signal to the voltage controller 132 for controlling theillumination to the silicon substrate 20. This in turn alters theetching current, changing the shape of the pore and/or trench. Asdescribed above, according to the etching system 130, by controlling theillumination, the etching current can be kept constant to form poresand/trenches in a precise manner.

The feedback circuit 136 may be electrically connected to the positioncontroller of the polarizing filter. In this instance, the polarizingfilter is rotated according to a signal from the feedback circuit 136,allowing the illumination to the silicon substrate to be controlled in aprecise manner.

Seventh Embodiment

Although in the first embodiment the grid metal layer is integrallyformed on the back surface of the silicon substrate, it may be formed asan independent member capable of being separated from the silicon wafer.Specifically, FIG. 10A shows a grid metal plate 140 made of anelectrically conductive material and FIG. 10B shows a detail view of thegrid metal plate. Preferably, as described above, the size of the grid142 and of the openings 144 in the grid metal plate 140 are determinedso that the size of the grid 142 is smaller than the thickness of thesilicon substrate.

The grid metal plate 140 is sealingly attached on the back surface ofthe silicon substrate and then secured by a suitable fixing member.Preferably, the grid metal plate 140 is manufactured integrally with thefixing ring which is used for fixing the silicon substrate to theetching bath. This facilitates the fixing of the grid metal plate 140 tothe silicon substrate without any difficulty.

Eighth Embodiment

FIG. 11 shows an accelerometer or acceleration sensor 150 manufacturedby the use of the electrochemical etching system of the presentinvention. The acceleration sensor 150 has a base 152. The base 152 hasa wall 154 extending vertically from the base 152. The wall 154 has aplurality of spaced, cantilever-like, deformable portions 156 extendingin parallel from the wall 154. The base 152, wall 154, and deformableportions 156 are formed in a single product 158 by shaping a siliconsubstrate using the electrochemical etching technique of the presentinvention. Each of the deformable portions 156 supports a strain member,e.g., piezoelectric member or resistance 160, for measuring adeformation of the deformable portion 156.

In operation, when any acceleration is acted on the acceleration sensor150, each of the deformable portions 156 sags toward a directionopposing to the acceleration. Then, the deformation of the deformableportion 156 is detected from the change of resistance of thepiezoelectric member 160.

He Referring to FIGS. 12A to 12E, a process for manufacturing theproduct 158 from the silicon substrate will be described below. Atfirst, prepared is an n-type silicon substrate 162 with a certainthickness (see FIG. 12A). Then, a silicon nitride layer 164 is depositedon one surface of the n-type silicon substrate 162 by the chemical vapordeposition, for example. Next, portions of the nitride layer are removedby the photolithography to form patterning grooves 166 for etching thecorresponding portions of the silicon substrate to define the tip endsurface and the side surfaces of each of the deformable portions 156.Subsequently, as shown in FIG. 12B, portions of the silicon substrate,exposed in the patterning grooves 166, are etched by wet etching with asuitable alkaline solution or reactive ion etching, for example, formingpits 168 from which the subsequent etching will be initialized.

The silicon substrate 162 with the nitride silicon layer 164 is mountedon the electrochemical etching system for its etching. At this moment,the silicon substrate 162 is positioned so that the nitride siliconlayer 164 and the patterning grooves 166 opposes to the etchingelectrolyte, i.e., hydrofluoricacid, and light, is illuminated to theback surface of the silicon substrate 162. The opposing electrode may bemade of platinum.

The etching is performed in two steps. In the first etching process, theplatinum electrode and the silicon substrate are biased so that thesilicon substrate is +2 volts higher than the platinum electrode. Anaverage illumination is set 70 W/cm². The ratio of the maximum andminimum illumination is set 1.69 or less. With this condition, as shownin FIG. 12C, the silicon substrate 162 is etched to form verticaltrenches 170 extending toward the back surface of the silicon substrate162, corresponding to the patterning grooves 166. This first etchingprocess is continued for about 15 minutes.

In the second etching process, the average illumination is increased to200 W/cm². Other conditions are the same as those in the first etchingprocess. As a result, as shown in FIG. 12D, the leading end portion ofthe vertical trenches 170 are extended laterally to form lateraltrenches 172, which results in the individual deformable portions 156.

Finally, if necessary, the nitride silicon layer 164 is removed byetching, for example. Although one product 158 is illustrated in FIGS.11 and 12A-12E for clarity, a number of products 158 may be formedsimultaneously in one silicon substrate. In this instance, after thepiezoelectric resistances are provided on the deformable portions, theacceleration sensors are divided into pieces by dicing, for example.

Also, for the acceleration sensor, a typical thickness of the deformableportion may be about 20 μm. However, it may be varied to obtaindifferent acceraletion sensors with different sensitivities. Simply bycontrolling the etching period for the first and/or the second etching,the thickness can be varied.

As described above, by the use of the electrochemical etching method, anumber of sensors can be manufactured in one wafer. Also, a product witha complicated structure can be made through one etching including firstand second etching processes, reducing the manufacturing time and costof the products considerably.

Naturally, the above etching is used in the manufacturing not only ofthe acceleration sensors but also of other devices including productswith a complicated shape.

Ninth Embodiment

FIG. 13 shows a light conducting member 180 manufactured by theelectrochemical etching system of the present invention. The lightconducting member 180 has a product 182 made of one n-type siliconsubstrate. In this embodiment, the product 182 is in the form ofsubstantially rectangular plate including a first region or latticestructure 184 in which a number of small pores are formed at regularintervals (e.g., in a lattice) and a light conducting passage 186running across the lattice structure 184. Although the light conductingpassage 186 extends in the form of L from one side surface 88 to anotherside surface 190, it is not limited thereto.

The light conducting member 180 uses a feature of the lattice structure188, which selectively eliminates light having a wavelengthcorresponding to the pitch of the pores. The feature is introduced inthe Journal of Applied Physics, Vol. 66 25, pp. 3254-3256. With thislight conducting member 180, when light 192 is guided into an one end ofthe light conducting passage 186 in the side surface 188, light 194except for a part of light having a wavelength corresponding to the sizeof the lattice is transmitted through the lattice structure 184. Thepart of light 196, which is prohibited from passing through the latticestructure 184, is guided by the light conducting passage 186 and thenfed out of the other end of the passage 186 in the side surface 180. Asdescribed above, only the part of light having the specific wavelengthis selectively extracted.

Referring next to FIGS. 14A to 14E, descriptions will be made to aprocess for manufacturing the product 182 from a silicon substrate.First, an n-type silicon substrate 200 having a certain thickness isprepared (FIG. 14A). Then, a nitride silicon layer 202 is deposited onone surface of the n-type silicon substrate 200 by the chemical vapordeposition (CVD), for example. Next, portions of the layer,corresponding to the pores in the lattice structure 184, are removed bya photolithography, for example, forming a pattern of pores. In thedrawing, concave portions formed by the removal process are indicated at204. An interval of the concave portions 204 is about 700 μm. However,the interval may vary depending upon a wavelength of light to beseparated. Subsequently, as shown in FIG. 14B, by an wet etching usingalkaline solution or a reactive etching, parts of the silicon substrateexposed in the concave portions are formed with pits 206 where asubsequent etching will be initiated therefrom.

The silicon substrate 200 with the nitride silicon layer 202 is mountedon the electrochemical etching system for the etching of the siliconsubstrate 200. At this moment, the silicon substrate 200 is positionedso that the nitride silicon layer 202 and the concave portions 204oppose the electrolyte, i.e., hydrofluoricacid, so that light isilluminated to the opposite surface of the silicon substrate 200.Platinum is used for the opposing electrode.

The etching is performed in two steps. In the first etching process, theplatinum electrode and the silicon substrate are biased so that thesilicon substrate is +2 volts higher than the platinum electrode. Anaverage illumination is set 40 W/cm². The ratio of the maximum andminimum illumination is set 1.69 or less. This causes that, as shown inFIG. 14C, the vertical pores 208 each having a depth of about 100μm areextended in the silicon substrate 200, corresponding to the porepattern.

Before the second etching process, the silicon substrate 200 is removedfrom the electrochemical etching system. As shown in FIG. 14D, thenitride silicon layer is eliminated from the silicon substrate 200 andthen a metal layer, e.g., aluminum layer 210, is deposited by thephysical or chemical vapor deposition, e.g., sputtering, instead. Also,a part of the aluminum layer corresponding to the light conductingpassage 186 is removed.

In the second etching process, portions of the silicon substrate 200,not coated with the aluminum layer 210, are etched by the reactive ionetching. This results in the lattice structure 184 with pores 208 andthe light conducting passage 186 running across the lattice structure184.

With the light conducting member 180 in which pores are formed in alattice, light having a wavelength of 1.5 μm is selectively extracted.Another light conducting member capable of extracting light with adifferent wavelength is obtained by changing the interval and/or thesize of the pores. Also, by the use of the above-describedelectrochemical etching method, another light conducting member having alarger area can be obtained.

Although there has been described several embodiments in accordance withthe present invention, it will be appreciated that the invention is notlimited thereto. Accordingly, any and all modifications, variations, orequivalent arrangements which may occur to those skilled in the artshould be considered to be within the scope of the present invention asdefined in the appended claims.

What is claimed is:
 1. An electrochemical etching system, comprising: acontainer for an etching bath and for holding an n-type siliconsubstrate so that a first surface of said silicon substrate contactshydrofluoric acid in the etching bath; an electrode positioned in thehydrofluoric acid; a power source having a positive terminal connectedto the silicon substrate and a negative terminal connected to theelectrode; and an illumination unit having a light source forillumination of a second surface of the silicon substrate with anillumination intensity of at least 10 mW/cm² and including aillumination controller for controlling the illumination of the secondsurface of the silicon substrate.
 2. The electrochemical etching systemin accordance with claim 1, wherein a ratio of a maximum illumination toa minimum illumination of the second surface of the silicon substrate isno more than 1.69:1.
 3. The electrochemical etching system in accordancewith claim 1, further comprising: a reference electrode positioned inthe hydrofluoric acid; and a voltage meter electrically connectedbetween said reference electrode and the silicon substrate.
 4. Theelectrochemical etching system in accordance with claim 1, wherein saidillumination controller controls quantity of light emitted from saidlight source.
 5. The electrochemical etching system in accordance withclaim 1, wherein said illumination controller includes a modulator, saidmodulator being connected between said light source and the siliconsubstrate for modulating the light emitted from said light source. 6.The electrochemical etching system in accordance with claim 1, furthercomprising: a current detector for detecting an electric currentsupplied from said power source to the silicon substrate; and anelectric circuit for controlling quantity of the light emitted from saidlight source based upon the electric current detected by said currentdetector.
 7. The electrochemical etching system in accordance with claim1, further comprising a unit for retaining a stable quality of thehydrofluoric acid.
 8. An electrochemical etching system comprising: acontainer for an etching bath and for holding an n-type siliconsubstrate so that a first surface of said silicon substrate contactshydrofluoric acid in the etching bath; an electrode positioned in thehydrofluoric acid; a power source having a positive terminal connectedto the silicon substrate and a negative terminal connected to theelectrode; an illumination unit having a light source for illuminationof a second surface of the silicon substrate with an illuminationintensity of at least 10 mW/cm²; and a metal plate positioned on thesecond surface of the silicon substrate, said metal plate having aplurality of openings arranged uniformly for transmitting the lightemitted from said illumination unit toward the second surface of thesilicon substrate.
 9. The electrochemical etching system in accordancewith claim 8, wherein said metal plate is electrically conductive andmounted on the second surface of the silicon substrate.
 10. Theelectrochemical etching system in accordance with claim 9, wherein saidmetal plate is integrally formed on the second surface of the substrate.11. The electrochemical etching system in accordance with claim 9,wherein said metal plate is independently formed on the second surfaceof the substrate.
 12. The electrochemical etching system in accordancewith claim 8, wherein a part of said metal plate remaining betweenneighboring openings has a width larger than a thickness of the siliconsubstrate.
 13. An electrochemical etching method comprising: placing afirst surface of an n-type silicon substrate in contact with anelectrolyte, arranging a metal plate on a second surface of the siliconsubstrate, the metal plate having a plurality of openings arrangeduniformly, illuminating the second surface of the silicon substratethrough the openings with an illumination intensity of at least 10mW/cm², and controlling an etching current with the illumination of thesecond surface to form a pore or trench in the first surface of thesilicon substrate.
 14. The electrochemical etching method in accordancewith claim 13, wherein a ratio of a maximum illumination to a minimumillumination of the second surface of the silicon substrate is no morethan 1.69:1.
 15. A product manufactured by the electrochemical etchingmethod in accordance with claim
 13. 16. An electrochemical etchingmethod comprising: placing a first surface of an n-type siliconsubstrate in contact with an electrolyte; illuminating a second surfaceof the silicon substrate with a first illumination intensity of at least10 mW/cm², controlling an etching current with the illumination of thesecond surface to form pores or trenches in the first surface of thesilicon substrate extending toward the second surface of the siliconsubstrate; and thereafter illuminating the second surface of the siliconsubstrate with a second illumination intensity, higher than the firstillumination intensity, to extend the pores or trenches laterally toconnect the pores or trenches to each other.